1. Field of the Invention
The present invention relates to an improved process for preparing unsaturated carboxylic acid esters and a novel apparatus for preparing the same. More particularly, the present invention relates to a process for esterification of an (aliphatic) alcohol having 1 to 8 carbon atoms and an unsaturated carboxylic acid by means of a cation exchange resin catalyst to prepare a corresponding unsaturated carboxylic acid ester, which provides a high yield of the desired ester product, inhibits the production of polymer by-products at the maximum level, maintains the conversion of the reactants at a constant rate regardless of the activity of catalyst and extends the life of catalyst.
2. Background Art
In the prior art, numerous methods have been utilized for esterification of said alcohol and unsaturated carboxylic acid to prepare a corresponding unsaturated carboxylic acid ester. However, those methods have some disadvantages in that a rate of conversion into an ester is low due to a low reaction rate and a relatively low equilibrium constant, and further the yield of ester products is low because etherification of alcohol and/or polymerization of (meth)acrylic acid may occur as a side reaction during esterification reaction.
A commercial method for preparing unsaturated carboxylic acid esters which is currently used in industrial scale is a method wherein a glass-lined batch reactor is used, an acid catalyst such as sulfuric acid or p-toluenesulfonic acid is used to increase the reaction rate and water produced during the reaction is continuously removed to induce the forward reaction. However, this method has some disadvantages in an economical view and an amount of waste water since the reactor is very expensive and the catalyst used in this reaction should be neutralized with a base such as NaOH.
Recently, some attempts have been made to provide a continuous process having an improved economics by using a strong cation exchange resin, which is formed by binding a sulfonic acid group (SO.sub.3 H) to a copolymer of polystyrene and DVB (divinyl benzene), as a catalyst for preparing said unsaturated carboxylic acid esters.
Particularly, Japanese Laid-open Patent Publication No. (sho) 49-54,326 discloses a reactor in the form of a fluidized bed, wherein the catalyst is suspended in the reactant by blowing a large amount of inert gas into the bottom of the reactor. However, this type of reactor has numerous disadvantages in that the vaccum condition cannot be easily maintained when the pressure within the reactor is reduced for extracting water produced during the reaction, and further the use of a large amount of gas has no economical merit.
In addition, Japanese Laid-open Patent Publication No. (sho) 63-17,844 discloses the use of a reactor in the form of a CSTR (continuous stirred tank reactor) for said esterification reaction. However, although this CSTR type of the reactor is useful in view of transfer of the reactants in the reactor and suspension of the catalyst, it has some disadvantages in that due to a weak mechanical strength of the cation exchange resin catalyst the catalyst can be readily broken and therefore the catalytic activity is lowered.
Further, Japanese Patent Publication No. (sho) 62-39,150 discloses a fixed bed-type reactor having an outer jacket, which can increase the reaction yield by vaporizing and extracting water produced during the reaction. However, the method using such type of reactor has also some disadvantages in that the catalyst present near the reactor wall can be deactivated with heat, the high temperature of the reactor wall dramatically enhances the polymerization reaction to produce a polymer of (meth)acrylic acid and its ester and further it is difficult to extract water produced during the reaction.
The present inventors have extensively studied to improve the disadvantages involved in the prior methods and reactors and found that in preparing unsaturated carboxylic acid esters by esterifying an (aliphatic) alcohol having 1 to 8 carbon atoms with an unsaturated carboxylic acid in the presence of a cation exchange resin catalyst, a method wherein water produced during the reaction can be continuously extracted to induce the forward reaction, the chances and times to contact the reactants with the cation exchange resin catalyst are increased and the area in which the catalyst is present is not directly heated, can be used to obtain a high yield of the desired product, a maximum inhibition of polymer production, a uniform conversion rate regardless of catalyst activity and a maximum extension of a life of the catalyst. Thus, we have completed the present invention.
Accordingly, it is an object of the present invention to provide an improved process for preparing unsaturated carboxylic acid esters from an (aliphatic) alcohol having 1 to 8 carbon atoms and an unsaturated carboxylic acid.
It is a further object of the present invention to provide a process for continuously preparing unsaturated carboxylic acid esters by esterifying an (aliphatic) alcohol having 1 to 8 carbon atoms with an unsaturated carboxylic acid in the presence of a cation exchange resin catalyst, characterized in that the process is conducted by continuously circulating said reactants in contact with the catalyst from the top to the bottom through a reactor which is composed in the form of a fixed bed divided into 1 to 10 steps wherein each step is composed of a catalyst bed, a filter and a air inlet, and has a thermal insulating outer wall, supplying a vaporizing heat for extraction of water, which is produced during the reaction, through a heat exchanger outside the reactor, circulating the reactants between the reactor and the heat exchanger by means of a circulating pump, contacting an azeotropic composition of water vaporized by the heat exchanger and an alcohol with an alcohol circulating in a column tower or a raw alcohol supplied from an alcohol inlet located on the top of a column tower to recycle the high boiling component into the reactor and at the same time to transfer the light component to the upper part of the column in which the light component is condensed, separating the condensate into an organic layer and water in a decanter, and then recycling the organic layer into the reactor and removing the water produced during the reaction.
Further, it is another object of the present invention to provide a novel apparatus suitably designed for carrying out the process of the present invention as defined above.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more pertinent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a more thorough understanding of the invention may be had by referring to the disclosure of invention and the drawing, in addition to the scope of the invention defined by the claims.